Crystal selector pattern

ABSTRACT

A crystal selector pattern for use in forming a mould in an investment casting process, which is formed from a water-soluble organic composition which, in a preferred embodiment, comprises 80% urea.

[0001] This invention relates to a crystal selector pattern for use inthe preparation of investment casting moulds for single crystal castingprocesses.

BACKGROUND OF THE INVENTION

[0002] In a conventional investment casting process, wax patterns of thearticle to be cast are assembled with wax patterns of other mouldcomponents, such as runner systems, and dipped in a ceramic slurrycomposition repeatedly, with each layer of slurry being dried before thenext dipping. The wax is then removed from the hardened slurry mould,for example by steam autoclaving, leaving a mould cavity of the desiredstructure. After firing in a kiln, molten metal is poured into the mouldcavity to form the components. The mould is then broken to release theformed components.

[0003] In a single crystal casting process, the mould is placed over achill plate, so that the molten metal solidifies first at the chillplate and the component is formed by solidification as a single grain inthe direction away from the chill plate. However, in practice, themolten metal cooled by the chill plate forms a large number of columnargrains, and so provision needs to be made to ensure that only one ofthese grains propagates into the mould cavity of the component itself soas to grow in the mould cavity as a single crystal. To achieve this acrystal selector is provided in the mould to annihilate most of thecolumnar grains generated from the furnace chill plate, leaving only onegrain to penetrate into the mould cavity. Such selectors comprise ahelical passage. The columnar grains growing into the selector passagemust branch in order to grow within the helix and consequently theyquickly annihilate until a single grain remains which grows along theremainder of the selector passage and into the mould cavity.

[0004] The selector works efficiently if the helical passage is narrow.However, a pattern to form such a narrow helical passage is verydelicate and the use of wax for this purpose is unsatisfactory becausethe wax is not strong enough to support the weight of the componentpatterns. Instead of wax, polystyrene has been used, but this has thesignificant disadvantage that it is not removed during the steamautoclave wax pattern removal process. Instead, the polystyrene isremoved during firing of the mould at high temperature, but the problemhere is that the polystyrene pattern expands more than the mouldmaterial and so has a tendency to crack the mould. The disadvantages ofwax and polystyrene selector patterns have inhibited the development ofsingle crystal casting using crystal selectors.

[0005] It is an object of the present invention to provide a crystalselector pattern which is sufficiently robust for reliable use in aninvestment casting process.

[0006] It is a further object of the present invention to provide acrystal selector pattern which is easily flushed from a mould cavity.

SUMMARY OF THE INVENTION

[0007] According to the present invention there is provided a crystalselector pattern for forming a crystal selector in an investment castingmould, the crystal selector pattern being formed from a water-solubleorganic composition.

[0008] Preferably, the water soluble organic composition is mouldableand so may be thermoplastic.

[0009] Preferably, the solubility of the composition is not less than500 g/l, and more preferably it is not less than 900 g/l.

[0010] The water-soluble organic composition may comprisehydroxypropylcellulose, but in a particular preferred embodiment, thecomposition comprises urea. Preferably, the urea comprises not less than50% of the composition, and more preferably not less than 70%.

[0011] The composition may also comprise a hydrophilic soluble polymersuch as polyvinyl acetate, which may be present in the composition inthe proportion not less than 10% and preferably not less than 15%.

[0012] The composition may also comprise an additive to enhance thedimensional stability of the crystal selector. Such an additive may, forexample, comprise a lubricant such as stearic acid, which may be presentin proportions of not less than 1%.

SUMMARY OF THE DRAWINGS

[0013]FIG. 1 represents an investment casting process; and

[0014]FIG. 2 shows a crystal selector pattern for use in the processrepresented in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0015]FIG. 1 shows mould cavities 2 for casting turbine blades of a gasturbine engine. The mould cavities 2 are present in a shell of ceramicmaterial (not shown) which also includes a runner system 4 and a pouringbasin 6 for introducing molten alloy into the mould cavity 2. At thelower end of each mould cavity 2, there is a crystal selector passage 8which is of helical form. The mould is positioned over a water cooledchill plate 10, with a starter block or seed crystal 12 positionedbetween the chill plate 10 and the lower end of each crystal selectorpassage 8.

[0016] In use, molten alloy is poured into the pouring basin to fill themould cavities 2 and other interior parts of the mould. The molten alloyreaching the cooled seed crystals 12 is itself rapidly cooled and beginsto solidify in the form of columnar grains extending upwardly from theseed crystals 12. These grains grow upwardly into the crystal selectorpassages 8. To propagate along the passages 8, the grains need to branchsuccessively and, in this process, they rapidly annihilate leaving onlya single grain growing from the upper end of the crystal selectorpassage 8. A single grain continues to grow into the mould cavity 2,with the result that the eventual blade is formed from a single crystal.

[0017] The mould is formed by assembling together patterns of thevarious cavities within the mould. Thus, wax patterns of the blades tobe formed in the mould cavities 2 are assembled with wax patternscorresponding to the runner system 4 and pouring basin 6. However, ifwax is used to form the crystal selector passages 8, the resultingpatterns have proved to be inadequately robust to withstand the stressesapplied to the patterns during the manufacture of the mould itself.Consequently, in accordance with the present invention, the patternsused for forming the crystal selector passages are made from athermoplastic injection mouldable water-soluble organic compositioncomprising, in a preferred embodiment, 80% urea, 18% polyvinyl acetateand 2% stearic acid.

[0018] The assembled patterns are dipped in a ceramic slurry material sothat a layer of slurry is formed over the patterns. Once this layer hasdried, the assembly is dipped again into the ceramic slurry, and so on,so that a series of layers is built up over the patterns. When the totalslurry layer is of adequate thickness, and is dried, the patterns areremoved from the interior by steam autoclaving. This raises thetemperature of the wax used for the patterns for the mould cavities 2,the runner system 4 and the pouring basin 6, so the wax melts and flowsfrom the mould cavities. The crystal selector patterns 8, being madefrom a water soluble material, are dissolved in the autoclaving steamand are similarly removed from the mould cavity. When the mould cavityis clean, the mould itself is fired in a kiln before use in the castingprocess.

[0019] The use of urea for the crystal selector pattern results in acomposition of very high solubility, urea having a solubility in excessof 1000 g/l. However, urea is brittle and the addition of polyvinylacetate serves to improve the toughness of the composition. Stearic acidserves to improve the dimensional stability of the resulting moulding,as well as improving the injection moulding properties of thecomposition. Dimensional stability is important in order to avoidcracking of the mould by dimensional changes in the crystal selectorpattern which result, for example, from temperature changes.

[0020] The present invention thus provides a crystal selector patternwhich is suitable for use in an investment casting process, the selectorpattern being made from a composition which can readily be formed intothe desired shape by injection moulding, and which is easily removedfrom the moulding cavity by conventional processes used for removing waxpatterns.

I claim:
 1. A crystal selector pattern for forming a crystal selector inan investment casting mould, the crystal selector pattern being formedfrom a water-soluble organic composition.
 2. A crystal selector patternas claimed in claim 1, in which the water-soluble organic composition isthermoplastic.
 3. A crystal selector pattern as claimed in claim 1,which is formed in an injection moulding process.
 4. A crystal selectorpattern as claimed in claim 1, in which the solubility of thewater-soluble organic composition is not less than 500 g/l.
 5. A crystalselector pattern as claimed in claim 4, in which the solubility of thewater-soluble organic composition is not less than 900 g/l.
 6. A crystalselector pattern as claimed in claim 1, in which the water-solubleorganic composition comprises hydroxypropylcellulose.
 7. A crystalselector pattern as claimed in claim 1, in which the water-solubleorganic composition comprises urea.
 8. A crystal selector pattern asclaimed in claim 1, in which the water-soluble organic compositioncomprises polyvinyl acetate.
 9. A crystal selector pattern as claimed inclaim 1, in which the water-soluble organic composition comprisesstearic acid.
 10. A crystal selector pattern for forming a crystalselector in an investment casting mould, the crystal selector patternbeing formed from a water-soluble organic composition which containsurea.
 11. A crystal selector pattern as claimed in claim 10, in whichurea is present in the water-soluble organic composition in a proportionnot less than 50%.
 12. A crystal selector pattern as claimed in claim11, in which the urea is present in the water-soluble organiccomposition in a proportion not less than 70%.
 13. A crystal selectorpattern as claimed in claim 10, in which the water-soluble organiccomposition comprises polyvinyl acetate.
 14. A crystal selector patternas claimed in claim 13, in which the polyvinyl acetate is present in thewater-soluble organic composition in a proportion not less than 10%. 15.A crystal selector pattern as claimed in claim 14, in which thepolyvinyl acetate is present in the water-soluble organic composition ina proportion not less than 15%.
 16. A crystal selector pattern asclaimed in claim 10, in which the water-soluble organic compositioncomprises stearic acid.
 17. A crystal selector pattern as claimed inclaim 16, in which the stearic acid is present in the water-solubleorganic composition in a proportion not less than
 1. 18. A crystalselector pattern for forming a crystal selector in an investment castingmould, the crystal selector pattern being formed from a water-solubleorganic composition which comprises: 70 to 90% urea, 10-25% polyvinylacetate, and 1 to 5% stearic acid.
 19. A crystal selector pattern inaccordance with claim 18, in which the water-soluble organic compositioncomprises: 80% urea, 18% polyvinyl acetate, and 2% stearic acids.
 20. Aninvestment casting process comprising: making a pattern corresponding tothe shape of a desired mould cavity, which mould cavity includes acrystal selector region; investing the pattern with a hardenable coatingof a ceramic material thereby to form a mould cavity about the pattern;hardening the coating; removing the pattern from the mould cavity; andcasting an article in the mould cavity, wherein the patterncorresponding to the crystal selector region is made form awater-soluble organic composition, and wherein the step of removing thepattern corresponding to the crystal selector region comprises steamautoclaving the moulding cavity.
 21. An investment casting process asclaimed in claim 20, wherein the water-soluble organic compositioncontains urea.
 22. An investment casting process as claimed in claim 20,wherein the water-soluble organic composition comprises: 70 to 90% urea,10-25% polyvinyl acetate, and 1 to 5% stearic acid.
 23. An investmentcasting process as claimed in claim 20, wherein the water-solubleorganic composition comprises: 80% urea, 18% polyvinyl acetate, and 2%stearic acid.